In an electronic device in which a circuit board is incorporated in a metal chassis, a Frame-Ground connection (FG connection) of the circuit board to the chassis is frequently performed. In the FG connection, a ground pattern is provided in the circuit board in order to stably operate a circuit in the circuit board, and the ground pattern is electrically connected to the chassis.
FIG. 7 schematically illustrates an example of the FG connection in the electronic device.
Referring to FIG. 7, an electronic device 500 includes a chassis 510 and a circuit board 520. The chassis 510 includes a metal box 511 and a metal cover 512, and the circuit board 520 is incorporated in the chassis 510.
Plural electronic components 521 are mounted on the circuit board 520, and a circuit is formed by the electronic components 521 and wiring patterns (not illustrated). In the circuit board 520, ground patterns 522 corresponding to a ground of the formed circuit are formed at four corners and in the center which is close to areas where the electronic components 521 are mounted in the surface of the circuit board 520.
In the circuit board 520, each of the four corners is tightened to each of board fixed sections 511a with a screw 530, and the center is tightened to a boss with a screw 540. The board fixed sections 511a are provided at the four corners of the box 511, and the boss is vertically provided from a bottom of the box 511 toward the circuit board 520. Therefore, the circuit board 520 is fixed to the box 511 while the ground pattern 522 is electrically connected to the box 511, and the FG connection of the circuit board 520 to the chassis 510 is performed.
FIG. 8 schematically illustrates a state in which an area where the ground pattern is formed is tightened in the circuit board of FIG. 7.
FIG. 8 typically illustrates the state in which the area where the ground pattern 522 is formed is tightened in the center of the circuit board 520.
As illustrated in FIG. 8, a boss 550 is vertically provided toward the circuit board 520 in a bottom 511b of the box 511 of FIG. 7, and the area where the ground pattern 522 is formed in the circuit board 520 is tightened to the boss 550 with the screw 540. In FIG. 8, the electronic component 521 of FIG. 7 is not illustrated for the sake of simple illustration.
In the example of FIGS. 7 and 8, the five ground patterns 522 are tightened to the box 511 with the screws 530 and 540, that is, the five ground patterns 522 are electrically connected to the chassis 510 of the electronic device 500 of FIG. 7, thereby performing the FG connection of the circuit board 520 to the chassis 510.
Recently a mobile electronic device such as a notebook type personal computer (notebook PC) is widely spread. In the electronic device such as the notebook PC, the FG connection of the built-in circuit board to the chassis is frequently performed by the structure illustrated in FIGS. 7 and 8.
At this point, in the electronic device such as the notebook PC, an external force is frequently applied to the chassis during carry. The chassis of the electronic device such as the notebook PC, as illustrated in FIG. 7, has a low profile in which a low sidewall is sandwiched between the wide cover and bottom. As a result, the external force is frequently applied to a direction intersecting the cover and bottom. Because the sidewall acts as a beam in an edge portion of the chassis, the sidewall is resistant to the external force applied to the direction intersecting the cover and bottom, and a deformation caused by the external force can be prevented. However, the center portions of the cover and bottom are susceptible to the external force, and the deformation such as deflection is easily generated.
For example, when the deformation is generated in the center portion of the cover or bottom by the external force in the electronic device 500 of FIG. 7, the external force propagates to the area where the ground pattern 522 is formed in the circuit board 520 by way of the center boss 550 of FIG. 8 used in the FG connection, thereby generating the deformation of the circuit board 520.
Generally the FG connections of the circuit board to the chassis are desirably performed at plural areas of the circuit board. When the FG connections are performed at the plural areas, some of the FG connections are performed near the electronic component like the center of the circuit board of FIG. 7. Therefore, a serious trouble such as a breakage of connection portion of the electronic component to the circuit board is possibly raised, when the deformation is generated near the electronic component of the circuit board by the external force propagating to the area where the ground pattern is formed.
In order to avoid the generation of the trouble, for example, the following FG connection is performed using the ground pattern which is formed while avoiding the neighborhood of the electronic component.
FIG. 9 schematically illustrates an example of the FG connection with the ground pattern which is formed while avoiding the neighborhood of the electronic component.
Referring to FIG. 9, similarly to the FG connection of FIG. 7, an electronic device 600 includes a chassis 610 and a circuit board 620. The chassis 610 includes a metal box 611 and a metal cover 612, and the circuit board 620 is incorporated in the chassis 610.
In the circuit board 620 on which plural electronic components 621 are mounted, ground patterns 622 are formed only at four corners while avoiding the neighborhood of the electronic component 621, and the FG connection is performed by tightening the circuit board 620 to the chassis 610 at the corners with screws 630. That is, in the example of FIG. 9, the FG connection is performed while avoiding the neighborhood of the electronic component 621 in the center of the circuit board 620.
Additionally, in the example of FIG. 9, a boss 640 is provided in order to strengthen the center portion of the cover 612 or the bottom of box 611. The boss 640 is vertically provided from the bottom of the box 611, and the boss 640 reaches the cover 612. The center of the cover 612 is tightened to the boss 640 with a screw 650. Therefore, in the circuit board 620, a through-hole 623 through which the boss 640 pierces is made in the center that is the neighborhood of the electronic component 621.
FIG. 10 is an enlarged view schematically illustrating surroundings of the through-hole in the circuit board of FIG. 7.
As illustrated in FIG. 10, because a diameter of the boss 640 is smaller than an inner diameter of the through-hole 623 in the circuit board 620, even if the deformation is generated in the center of the cover 612 or bottom 611b by the external force, the circuit board 620 is independent of the deformation. Therefore, the trouble such as the breakage of the connection portion of the electronic component 621 to the circuit board 620 of FIG. 9 can be avoided. However, in the example of FIGS. 9 and 10, because the ground pattern 622 used in the FG connection is formed while avoiding the neighborhood of the electronic component 621, the number of FG connections tends to be decreased, and a ground resistance of the circuit to the chassis 610 is increased to possibly deteriorate stability of a circuit operation.
Thus, the demand to realize the FG connections which are performed at as many areas as possible while suppressing the external force propagation to the built-in circuit board arises in the electronic device such as the notebook PC having the low-profile chassis that is susceptible to the external force applied in the center of the cover or bottom of the chassis.
For example, Japanese Laid-open patent Publication No. 2005-302789 discloses a technique of interposing a cushioning material having conductivity between the circuit board and the chassis-side boss used to tighten the circuit board to the chassis, and Japanese Laid-open patent Publication No. 2006-332515 discloses a technique of interposing a metal spring washer between the screw and the circuit board. A conductive resin such as a conductive elastomer is known as the cushioning material having the conductivity (for example, see Japanese Laid-open patent Publication No. 2004-72051). It can be considered that by applying the techniques disclosed in the above publications to the electronic device having the structure of FIGS. 7 and 8, the propagation of the external force applied to the cover or bottom of the chassis to the circuit board is suppressed by the cushioning material, so that the FG connection is performed near the electronic component to realize the FG connections at plural areas.
However, in the technique of interposing the cushioning material to suppress the external force propagation to the circuit board, there is a problem that the resistance of each FG connection is increased by the interposition, even though the cushioning material has conductivity.